Efficient distributed sensor fiber

ABSTRACT

A method and apparatus for improving the sensing of a physical parameter using a distributed optical waveguide and scattering. The optical waveguides have improved scattering efficiency and/or improved light capturing capability provided by multi-cladding layers and a tightly confining core waveguide. The core can be highly doped with a material such as germanium to improve scattering. The cladding layers provide a multi-mode waveguide for capturing scattered light. Such optical waveguides are useful in systems that rely on Rayleigh, Raman and Brillouin scattering.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of co-pending U.S. patent applicationSer. No. 10/862,004, filed Jun. 4, 2004, herein incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention relate to distributed opticalwaveguide sensors. More specifically, embodiments of the presentinvention relate to distributed optical waveguide sensors having opticalwaveguides with multiple cladding layers.

2. Description of the Related Art

Light propagating in a medium can undergo a variety of scatteringevents, both linear and non-linear. Three types of light scattering areRayleigh, Raman and Brillouin. In Rayleigh scattering, incident light iselastically scattered at the same wavelength. In Raman scattering,incident light is scattered by the vibrations of molecules or opticalphonons and undergoes relatively large frequency shifts. In Brillouinscattering, incident light is scattered by acoustic vibrations (phonons)and undergoes relatively small frequency shifts.

Rayleigh, Raman and Brillouin scattering can be used in distributedoptical waveguide sensors to measure a measurand such as temperature orstress over the length of an optical waveguide. Since optical waveguidescan be over 30 kilometers long, distributed optical waveguide sensorsare suitable for measuring physical parameters over large distances.Distributed optical waveguide sensors that use Rayleigh, Raman, orBrillouin scattering are typically based on either Optical Time-DomainReflectometry (OTDR) or optical frequency-domain reflectometry (OFDR).In either case, high intensity laser light is propagated in the core ofan optical waveguide. Light scattering occurs within the waveguide, partof which is captured in the backward propagating modes of the waveguideand can be detected by a receiver. By monitoring one or more variationsin the captured light a physical parameter can be determined.

While useful, distributed optical waveguide sensors based on scatteringhave problems because scattering produces signals that are much weakerthan the light that created them. In optical waveguides, the originatinglight, referred to as pump radiation, produces a relatively small amountof scattered light, only a portion of which is captured. Because thecaptured light is weak, a significant integration time is required toproduce measurements with suitable resolution and accuracy.

Therefore, an optical waveguide with improved scattering efficiencywould be useful. An optical waveguide that enables improved capture ofscattered light would also be useful.

SUMMARY OF THE INVENTION

Embodiments of the present invention generally provide for distributedoptical waveguide sensors having optical waveguides with improvedscattering efficiency and/or improved light capture.

Embodiments of the present invention comprise an optical waveguidehaving multiple cladding layers. Some embodiments have predominantlysingle-mode cores. Some embodiments have cores that are doped to improvescattering, e.g., highly germanium doped cores. Some embodiments includea first cladding layer and a second cladding layer.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the present invention can be understood in detail, a particulardescription of the invention, briefly summarized above, may be had byreference to embodiments, some of which are illustrated in the appendeddrawings. It is to be noted, however, that the appended drawingsillustrate only typical embodiments of this invention and are thereforenot to be considered limiting of its scope, for the invention may admitto other equally effective embodiments.

FIG. 1 is a schematic depiction of a distributed optical waveguidesensor system that is in accord with the principles of the presentinvention;

FIG. 2 schematically illustrates a section of an optical waveguide thatis in accord with the principles of the present invention;

FIG. 3 illustrates the refractive indexes of a double-clad opticalwaveguide according to one embodiment of the present invention; and

FIG. 4 illustrates the refractive indexes of a double-clad opticalwaveguide according to another embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides for distributed optical waveguide sensorshaving optical waveguides with improved scattering efficiency and/orwith improved scattered light capture. A distributed optical waveguidethat is in accord with the principles of the present invention hasmultiple cladding layers. In some embodiments a predominantlysingle-mode core, possibly highly germanium doped, provides improvedscattering efficiency. The multiple cladding layers provide for amultiple mode optical waveguide for improved light capture. It should beunderstood that the principles of the present invention will boostsignal levels for systems using either optical time domain reflectometry(OTDR) or optical frequency domain reflectometry (OFDR).

FIG. 1 schematically depicts a distributed optical waveguide sensorsystem 100 that is in accord with the principles of the presentinvention. As shown, the sensor system 100 includes a distributedoptical waveguide 102. That optical waveguide, which includes a core andmultiple cladding layers, is discussed in more detail subsequently. Thesensor system 100 includes a transmitter 104 and a receiver 106 that issuitable for use with optical time domain or optical frequency domainreflectometry. It is within the scope of the present invention thatreceiver 106 may comprise any number of individual components necessaryto produce or enhance the performance of the invention as describedherein. Such components include by way of example and not by limitation,a photo detector, a data analyzer, an analogue-to-digital converter, anamplifier, and other similar devices known by those skilled in the artto assist in the reception of light and its meaningful interpretation asset forth herein. Similarly, the transmitter 104 may comprise any numberof individual components necessary to produce or enhance the performanceof the invention as described herein. Such components include by way ofexample and not by limitation, a laser, a modulator, a controller, andother similar devices known by those skilled in the art to assist in thegeneration and transmission of light energy as set forth herein. Inaddition, the transmitter 104 and receiver 106 may be in communication(optically or electrically) as necessary for their operation.

FIG. 2 schematically illustrates a section of the optical waveguide 102.It should be understood that the optical waveguide 102 can be very long,with lengths of 1-30 kilometers being fairly common. As shown, theoptical waveguide 102 is comprised of a core 202, an inner claddinglayer 204, and an outer cladding layer 206. The core 202 is thin, has ahigh index of refraction (see FIGS. 3 and 4), and often only supports asingle transverse optical mode, although multiple modes may also besupported. As laser light 210 from the laser source/transmitter 104travels down the optical waveguide 102, the laser light 210 is scattered212 by the waveguide material. If the interaction 212 of the laser light210 and the waveguide material produces Rayleigh scattering the incidentlight is elastically scattered at the same wavelength. If theinteraction 212 is with an optical phonon the laser light 210 is Ramanscattered with relatively large frequency shifts. If the interaction 212is with an acoustic vibration (phonons) the laser light 210 is Brillouinscattered with relatively small frequency shifts. In any event, aportion of the scattered laser light 210 having suitable overlap withrespect to the propagating modes of the waveguide formed by the core202, the inner cladding layer 204 and the outer cladding layer 206 willbe recaptured by the optical waveguide 102.

The inner cladding 204 and outer cladding 206 form a multi-modewaveguide that efficiently transports the recaptured scattered light(along with the light recaptured by the core propagating modes) to thereceiver 106. That light is collected and processed to determine aphysical parameter of interest using known techniques. A highlymultimode waveguide having a large capture cross-section greatlyimproves the capture of the scattered light. While the optical waveguide102 is shown with two cladding layers, in some applications more thantwo claddings may be used.

Since distributed optical waveguides 102 operate by light scatteringwithin the core 202, it is beneficial to produce as much scattering aspossible. To that end, the pump radiation 210 should be confined in amode(s) with a small cross-section(s). This produces a high energydensity, which increases the scattering efficiency of the non-linearRaman and Brillouin scattering processes. Additionally, a single,well-confined core mode will generally produce the lowest attenuationand dispersion of the propagating laser light 210. As the length of adistributed optical waveguide 102 increases a well-confined core mode isparticularly useful. Core dopants and dopant concentrations, such ashighly doping the core 202 with germanium, or other dopants as is known,including rare-earth dopants, can increase scattering.

The refractive indexes of the optical waveguide 102 can be adjusted toimprove performance. FIG. 3 illustrates a refractive index profile of afirst embodiment optical waveguide, while FIG. 4 illustrates arefractive index profile of a second embodiment optical waveguide. Inboth figures, distance is shown on the X-axis (300 and 400) and therefractive index is shown on the Y-axis (302 and 402). The maximumrefractive index is in the core 202, shown as peaks 304 and 404, whilethe minimum refractive indexes, shown as lines 306 and 406, are in theouter cladding layer 206. In the embodiment shown in FIG. 3, therefractive index 308 of the inner cladding layer 204 is constant. Thus,the embodiment shown in FIG. 3 uses a step index. However, in theembodiment shown in FIG. 4, the refractive index 408 of the innercladding layer changes with radial distance. This can produce a betteroptical waveguide 102 in some applications.

More complex waveguide structures such as fibers with multiple rings ofdifferent refractive index or asymmetric transverse sections, waveguidesof different or multiple materials (e.g. glasses, liquids, gasses),planar waveguides, so-called ‘holey-fibers’ or photonic crystalstructures could all be designed to have the properties described inthis invention. A wave-guide portion enhances nonlinear scatteringthrough properties such as tight mode confinement, low loss and doping,and a waveguide portion enhances capture of the scattered light throughproperties such as large modal overlap with the scattered light and highnumber of guided modes.

The core of the waveguide structure does not necessarily have to beconcentric to the waveguide structure and may be positioned to optimizethe recapture of scattered radiation. The waveguide structure may evenconsist of multiple cores, one or more of which guide the pump radiationand one or more of which recapture the scattered radiation in accordancewith the principles already outlined.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

1. A distributed optical waveguide sensor apparatus, comprising: anoptical waveguide having a core and a cladding; a light source forinjecting light into said core, wherein said cladding captures andguides light scattered from said core; a receiver for converting guided,scattered light from said waveguide into electrical signals; and ananalyzer for determining a physical parameter of interest from saidelectrical signals.